The main reason why GLaDOS is obsessed with testing is they built a Need to test into the Disk Operatating System. She can't help but test... It's a need. That and the device (portal gun) was never proven safe, only the portals were.

she is testing for how a human mind solves problems in an attemp to copy it and so she does in Portal 2 where robots are to do harder tasks and the people are kept in pickle jars wired to Glados. It is all to weird for you to handle but is Area 51 stuff. Half Life 2 is the parallel sequal.